Various types of microactuators have been developed by microelectromechanical system (MEMS) technologies that use semiconductor processing. According to the MEMS technologies, a lot of actuators, driver circuits and so on can be made at the same time. Thus, their applications have been broadened so as to make full use of their advantageous features. A deformable mirror; which is an arrangement of a huge number of micromirrors on a substrate, is one of those applications. The deformable mirror has found applications in compensating optical systems, displays, optical communications and other apparatuses to actively correct the wave front aberration of light.
The following two methods are known as conventional methods of controlling the magnitude of displacement of a deformable mirror.
One method is an open loop control in which the displacement of the mirror is controlled with drive voltages applied in multiple stages (see R. W. Corrigan, D. T. Amm and C. S. Gudeman, “Grating Light Valve™ Technology for Projection Displays”, presented at the International Display Workshop, Kobe Japan, Dec. 9, 1998, Paper Number LAD 5-1, for example). In this document, the deformable mirror is used as a diffraction grating for controlling the amount of diffracted light with the magnitude of displacement of the mirror, which is subjected to an open loop control in multiple stages. This document also discloses the technique of correcting a variation in characteristic among a plurality of deformable mirrors by empirically tracing the relationship between the drive voltage and the amount of diffracted light at several points in advance during a manufacturing process and by compiling a conversion table with those relationships interpolated.
The other method is a closed loop control using an external sensor. In a compensating optical system, for example, a control signal for a deformable mirror is generated based on an error signal that has been detected with a wavefront sensor, thereby performing a closed loop control (see J. A. Perreault, T. G. Bifano et al., “Adaptive Optic Correction Using Microelectromechanical Deformable Mirrors”, Optical Engineering, Vol. 41, No. 3, pp. 561-566 (March, 2002), for example).
In the field of microsensors, the following technologies are known. Some pressure sensors sense the deformation of a diaphragm under an external pressure as a variation in electrostatic capacitance (see S. B. Crary, W. G. Baer et al., “Digital Compensation of High-Performance Silicon Pressure Transducers”, Sensors and Actuators, A21 to A23, pp. 70-72 (1990), for example). This document discloses a configuration in which relationships between the pressure and the sensor output are traced empirically in advance under multiple temperature conditions and in which a calibration polynomial, approximating these relationships, is stored in a memory.
There is also a force balanced pressure sensor, which makes another electrode produce electrostatic force to offset external pressure, performs a control so as to reduce the deformation of the diaphragm to substantially zero, and calculates the external pressure based on the magnitude of that electrostatic force (see B. P. Gogoi, C. C. Wang and C. H. Mastrangelo, “Force Balanced Micromachined Pressure Sensors”, IEEE Transactions on Electron Devices, Vol. 48, No. 8, pp. 1575-1584 (August 2001), for example).
Some angular velocity sensors sense the magnitude of displacement of a movable body due to the Coriolis force produced by an external angular velocity as a variation in electrostatic capacitance (see T. Juneau, A. P. Pisano and J. H. Smith, “Dual Axis Operation of a Micromachined Rate Gyroscope”, Transducers '97, 1997 International Conference on Solid-State Sensors and Actuators, Chicago, June 16-19, pp. 883-886). This document discloses a configuration for correcting the drift of a zero point due to the initial positional error of a movable body.
These microactuators, however, have the following drawbacks.
As for the microactuator for performing an open loop control by compiling a conversion table during the manufacturing process, it is troublesome to collect data to make the conversion table and the microactuator can cope with a variation with time or according to the environment only within a certain limit. For example, to trace a relationship between the drive voltage and the amount of diffracted light, the amount of light needs to be measured pixel by pixel by actually irradiating the microactuator with external light. Thus, a dedicated measuring device is needed and a lot of preparatory work, such as positioning the beam spot accurately, must be done. Consequently, it is very troublesome to collect the data required. Also, only the characteristic of the microactuator during the initial stage of the manufacturing process can be evaluated and the magnitude of displacement of the moving section cannot be monitored once the microactuator has been built in an apparatus actually. For that reason, even if the characteristic of the actuator has changed due to either a variation with time or a variation according to the environment (e.g., the temperature variation), corrections cannot be made appropriately so as to cope with such a change.
The microactuator for performing a closed loop control using an external sensor such as a wavefront sensor requires, first of all, an expensive control system. To carry out a closed loop control with good stability, the number of sensing points of the wavefront sensor needs to be greater than the number of actuators included in the deformable mirror. In a Shack-Hartmann type wavefront sensor, for example, it is generally believed that the number of sensing points needs to be at least twice as large as that of actuators. Accordingly, a sensor with relatively high resolution is needed to carry out the closed loop control. In addition, positioning adjustment has to be done so as to associate each sensing point of the wavefront sensor with a driving point of the deformable mirror precisely enough. Furthermore, a relatively high-precision and large-scale controller is needed to perform computations (such as wavefront reconstitution) on a plurality of sensing signals and generate control signals for respective driving points. Secondly, in that microactuator, a significant amount of light is lost by the wavefront sensor. The wavefront sensor senses the wavefront by using a portion of a bundle of rays, of which the wavefront should be corrected, thus leading to the loss of the amount of light. If the number of sensing points on the wavefront is increased for the purpose of the closed loop control and if the sensor needs to guarantee prescribed sensitivity (i.e., S/N ratio) for each sensing point, then the wavefront sensor causes a significant loss in the amount of light.
Furthermore, the microsensor such a pressure sensor or an angular velocity sensor has the following structural features and accompanying problems. First of all, the microsensor described in the document mentioned above senses and controls the displacement of just one movable body. However, if a plurality of actuators needs to be driven at the same time as in a deformable mirror, for example, and if the microsensor performs the closed loop control on each of those actuators, then the circuit size becomes extremely big for that purpose. That is to say, the same number of sensing signal generators, amplifiers, A/D converters, controllers and other circuits as that of the actuators are needed for the purpose of displacement sensing. Particularly when there are a huge number of actuators, the circuit scale increases tremendously and the overall chip cost rises significantly.
A second problem is that no configuration for tracing the relationship between the drive signal and the displacement and self-calibrating it is disclosed in that document. Thus, it is difficult to apply the conventional technique to increasing the displacement precision of the actuators. Both the pressure sensor and the angular velocity sensor include a movable body, which is displaced under externally applied force, and a structure for converting the displacement of this movable body into a sensor output. However, the correspondence prestored in a memory is used during this converting operation, and the correlation between the displacement of the movable body and the output is fixed except the zero point drift correction. The zero point drift correction is done to correct the offset while the movable body has not been displaced at all, and has essentially nothing to do with the relationship between the drive signal and the displacement. Accordingly, even if some mechanical property has varied with time (e.g., a spring constant has varied due to repetitive fatigue), that variation is not correctable.
That is to say, a configuration for self-calibrating the relationship between a drive signal for an actuator and the displacement thereof while displacing the actuator is disclosed in none of the documents cited above. Thus, it is difficult in the prior art to compensate for the actuator characteristic, which varies either with time or due to any of various environmental factors, in a broad displacement range.
In order to overcome the problems described above, an object of the present invention is to provide a microactuator and a deformable mirror, which realize high-reliability positioning adjustment with a simple configuration and with a variation in characteristic with time or according to the environment corrected.